(1) Histological tsudy of Bursa fabricus :Bursa Fabricus basically consists of inner epithelium, lamina propria mucosa, submucosa whithin developted lymph follicles, and capsule. At 0 day of hatching, size of the follicles was measured 100mum in small one, and the follicle cortex developed slightly. At 2-3 weeks of age, the size of the follicles became lager and 220mum, the follicle cortex developed fairly. At 6 weeks of age, the size of the follicles became lager than these of 4 or 5 weeks of age and 500mum, the follicle cortex developed and wave finding between cortex and medulla was observed. At 7 weeks of age, the size of the follicles became the largest one and 700mum, the follicle cortex developed well. At 9 weeks of age, the size of the follicles became smaller one and decreased to 400mum, the follicle cortex maintained the development. At 11-12 weeks of age, the size of the follicles became 150-400mum, the decrease of the follicle cortex was observed. At 14 weeks of age, the size of the follicles became 300mum, the follicle cortex became thin.These findings suggest that the deep relationship between the growth of the quail and the development ・ decrease of the lymph follicles was recognized.(2) Immunohistochemical study :B-cells were identified in the follicle by immunohistochemistry.(3) Identification of lymphocytes :Middle Iymphocytes and mitosis were observed in the follicle cortex and small or large cells were observed in the follicle medulla by transmission electron microscopy. The cells with microvilli were identified B-cells by scanning electron microscopy.These findings suggest that the identification of the cells was possible by immunohistological methods and electron microscopy.